In stacks of thermoformed parts, stack shoulders and other stacking devices are most effective if kept in vertical alignment within any given stack of parts. In many thin-walled thermoformed cups, for example, the stack device forms a continuous ledge around the cup. The cups are usually round and relatively small in diameter. This provides a minimal chance for stack shoulders to become misaligned.
Use of various types of stack shoulders including those of the reverse taper type is well known in the design of relatively thin-walled formed plastic articles such as drinking cups, dairy product containers, semi-rigid formed plastic meat packages and the like. The shape, size, location in part and degree of interference between stack ledges may vary widely since, for such relatively small parts, the undercut that can be pulled from commercial forming tools is sufficient to provide effective axial spacing control means. In items such as covers for plates however, the parts are relatively large with thin, relatively straight side-walls. In this type of large, flexible part, telescoping of one part within another is common and difficult to avoid for a number of reasons. For example, in parts that are nestably stackable, it is desired that minimal clearance between stacked parts be provided to reduce the stack height and thus reduce the space needed (and accompanying expense) during handling, shipping and storage of the parts. Although desirable, the amount that clearance can be reduced between parts and still permit satisfactory separation is limited. Such limits are imposed by several factors including the uniformity of material distribution in the sidewalls and the uniformity of the size and shape of parts being stacked. Additionally, as parts are separated from stacks, partial vacuum created between parts increases as clearance decreases and as the rate of separation increases making ready separation proportionately more difficult and often impossible. Another difficulty when using a reverse taper type shoulder, is that the undercut, i.e. those areas cut back or indented into the interior walls of the molds that correspond to the extended exterior portions of the stack shoulder, per side required to provide adequate resistance to telescoping between nested parts increases as 1) clearance between sidewalls of nested parts increases, 2) flexibility of sidewalls increases and 3) variation in dimensions and shape of nested parts increases. However, in relatively large, thin-walled flexible formed parts with stack shoulders placed near the open end of the sidewall, the undercut that can feasibly be produced at a reverse taper section of non-split forming tools is usually less than that required to provide effective telescope resistance. Since the amount of clearance between parts and the undercut permitted are also affected by variations in forming tools, process conditions and plastic material used, it will be appreciated that the production of such parts having adequate resistance to telescoping is imprecise and difficult to obtain.
Many proposals are present in the prior art directed to prevention of telescoping in stacked parts. Many of such proposals include stack ledges that are varied in shape, placement or spacing from one part to another in a stack or, where uniformly shaped and spaced, necessitate rotation of parts circumferentially in relation to adjacent parts to prevent telescoping. Where the undercut required is less than what can be feasibly produced on non-split commercial tooling, split tooling has been employed to permit stripping and removal of deeply undercut sections therefrom.